This invention relates to a method for manufacturing a porous polyetetrafluoroethylene material with high mechanical strength and more particularly to improvement in the economization of energy required for steps for removing a liquid lubricant, stretching and sintering the article.
The invention also releates to an apparatus for use in such method.
A porous polytetrafluoroethylene (hereinafter referred to "PTFE") material takes advantage of excellent heat resistance, chemical resistance, electric insulation and repellency of PTFE and is used in various filters, diaphragms and other waterproof gas-permeable materials, insulating materials for cables and sealing materials or the like.
Various methods for manufacturing a porous PTFE material have been well known. Among others, what is commercially appealing to the market is a stretching process for making porous PTFE whose basic idea is disclosed in Japanese Patent Publication No. 13560/67. In the first step of the process, powdered PTFE and a liquid lubricant are mixed and the mixture is shaped by a paste extrusion and a calender rolling or a combination thereof to obtain an unsintered molded article in the form of a film, tube or a rod and the like. The second and subsequent steps comprises: (1) eliminating the liquid lubricant contained in the molded article by evaporation or extraction, (2) rendering the molded article porous by strentching, and (3) sintering the porous article at a temperature higher than the melting point of PTFE to fix the porous structure. The reason why the second and subsequent steps are thus subdivided is as follows.
Stretching of molded PTFE containing a liquid lubricant at a temperature heretofore employed (from room temperature to a temperature lower than the crystalline melting point of PTFE) fails to lead to uniform stretching due to the action of interfacial tension of the liquid lubricant and give rise to a porous structure heterogeneous in pore size distribution. The sintering step must be carried out at a temperature higher than the crystalline melting point of PTFE. However, this step has been taken separately from the stretching step since the stretching has heretofore been carried out mostly at a temperature lower than the crystalline melting point of PTFE.
It has been customary to divide the process into unit steps to be carried out at different temperatures and different speeds, thus requiring much time and energy for processing and wasting much labor.